下面生成格雷编码的代码，横线处应填入：
for (string s : prev) result.push_back("0" + s);
for (________) result.push_back("1" + prev[i]);
- A. int i = 0; i < prev.size(); i++
- B. int i = prev.size()-1; i >= 0; i--
- C. auto s : prev
- D. int i = prev.size()/2; i < prev.size(); i++
正确答案：B